Introduction to Laser Safety

Lasers are unique in their safety hazards, particularly to something you
value highly - your vision. While the dangers of firearms and explosives are
obvious to most sane people, the possibility that a stream of massless photons
even from a low power laser can cause instant severe and irreversible damage
to vision or even total blindness is something that often needs to be stressed
and restressed. For high power lasers, there may be fire and other hazards as
well. And many lasers - even small ones - may use potentially lethal voltages.
There can be other dangers as well. If you don't read any other parts of
Sam's Laser FAQ, study the material that follows as well as the more specific
safety info in the chapters on each particular type of laser. Go to the
various laser safety Web sites to see how major institutions and regulatory
organization deal with laser safety. It is possible to work with lasers
safely and doesn't require rocket science - but it won't happen automatically.

WARNING: The information in this chapter should NOT be
considered a substitute for a comprehensive course in laser safety.
Casual reading and common sense precautions may be adequate when dealing with
low power visible CW lasers but is totally useless for anything above a few
milliwatts and for invisible or pulsed lasers, as accidents will
happen. And, if an accident means a beam in your eye, damage may very
likely be irreversible. As in permanent. As in, some portion of vision in
the affected eye(s) will be gone forever. Only classroom instruction with an
associated hands-on laser lab can develop and enforce the required procedures
and habits that will apply to a wide variety of laser equipment.

Lasers have tended to be high glamor devices popular with with hobbyists,
experimenters, entertainers, and serious researchers alike. However, except
for very low power lasers - those with less than a fraction of a mW of beam
power - they do pose some unique hazards particularly with respect to instant
and permanent damage to vision. The visual receptors (the light sensitive
cells) lining the eye's retina are part of the central nervous system and
do not regenerate. You're pretty much born with your lifetime allocation.

Here we only discuss the hazards with respect to vision. There are other
safety issues - such as the danger from the high voltages used to power
certain types of laser. These are summarized later in this chapter and dealt
with in more detail in the chapters on the lasers for which they apply.
There are several reasons that even small lasers which do not represent any
sort of burning or fire risk can instantly and permanently damage vision:

The output of many lasers is a nearly parallel - highly collimated - beam
which means that not only is the energy concentrated in a small area but the
lens of the eye will focus it to a microscopic point on the retina instantly
vaporizing tissue in much less than the blink of an eye. A collimated beam
represents the rays from an object at infinity so if your eye is focused for
distance, the laser will be in focus as well. Even a common helium-neon
laser without external optics will approximate a point source a .5 meter or
more behind the exit window of the laser. Where your are working in a small
room, this approximate distance would likely be where your eyes are focused.
While purists might argue that the lens of the eye isn't perfect and will
not produce a diffraction limited spot on the retina, this won't save your
vision! The power density in a sub-optimal spot can still be astronomical.

A cheap laser pointer also produces a highly collimated beam.

Even at power levels considered relatively safe, one shouldn't deliberately
stare into the beam for any reason. For these relatively low power lasers,
permanent eye damage is not that likely but why take chances? For these
lasers, viewing the spot projected on a white surface is perfectly safe.

A 100 W light bulb puts out about 5 to 7 W of visible light and another
35 to 40 W in the near-IR which is also relevant since it passes through
glass, water, and the anterior structures of the eye can be focused on the
retina. The rest is mid to far-IR and heat with a small amount of UV tossed
in. All of this radiation is more or less uniformly distributed in every
direction. However, at any reasonable distance from the light bulb, the
power density (e.g., W/mm2) entering the eye is much lower than
for a collimated laser beam of even very low power. And, it takes
significant effort to produce any sort of truly collimated beam from such a
non-point source such as is present with even the filament of a clear light
bulb. For a frosted light bulb, insert another factor of a thousand or so.
:) Without collimation, even the portion of that additional 35 to 40 W of
near-IR that enters the eye isn't going to cause damage. However, for a
helium-neon laser, the collimation is such that the entire beam (total power
output of the laser) will still be small enough to enter the eye even at a
distance of several meters.

For example, at 10 cm from a 100 W bulb (which would be a very uncomfortable
place to be just due to the heat), the power density of the visible light
(assuming 5 total watts) would be only about 0.05 mW/mm2. At 1 m,
it would be only 0.0005 mW/mm2 or 500 mW/m2. Based on
this back-of-the-envelope calculation, a 5 mW laser beam spread out to a
circular spot of 0.1 m diameter (i.e., 1 mR divergence at a distance of 100 m
- without external optics) will appear brighter than the 100 W light bulb at
1 m! And, close to the laser itself, that beam may be only 1 *mm* in
diameter and thus 10,000 times more intense! (And note that the other
invisible radiation that passes through to the back of the eye is still
not nearly as dangerous as the beam from the 1 mW laser because it isn't
focused to a tiny spot by the lens.)

As another point of reference, the mid-day Sun at the Earth's equator on a
clear day has a power density of about 1 kW/m2 or about
1 mW/mm2. It would not take very long staring into the Sun
to burn out your eyeballs! (Yes, I know, some people have claimed to do
this all day without harm - I wonder what a vision test would reveal?)
Also see the additional comparison, below.

See the section: Laser Safety Sites for
links to much more information on general laser safety, laser safety
organizations, and regulatory agencies.

And since laser pointers seem to be everywhere these days, consider this:
If carefully focused, as little as 5 or 6 mW from a laser is sufficient to
produce burn marks on black electrical tape along with wisps of smoke. Think
about what similar power levels can do to the delicate tissue at the back
of your eyeballs! While laser pointers themselves may not be quite as
dangerous as some people (and politicians) may have you to believe, that
such macroscopic effects can take place at these relatively modest power
levels should provide some additional respect for the damage that can result
under just the wrong set of conditions.

A popular graveyard joke in the laser industry is: "Do not stare into the
beam with your remaining good eye". Another one is: "How many times can I
look into a laser beam?". Answer: "Twice, once left, once right".
Or see Peer
Pressure in the Laser Lab from David Farley's
Doctor Fun Archive.
Nonetheless, laser safety is no laughing matter.

The power density of the HeNe laser on the retina is
1 mW/(6 x 10-5 mm2) = 16,667 mW/mm2
= 16.667 watts/mm2.

So the 1 mW laser has the potential to produce an intensity on the
retina 167 times that of direct sunlight! But there are many more
factors to consider in determining the real risk of damage. In
addition to those noted below, the actual focal point when looking at a laser
at close range will not be at the retina so the spot size will most likely be
much larger than the diffraction limit of the calculation. Even if the spot
from the laser beam is smaller, natural eye movements or movement of the
source (e.g., some moron waving a laser pointer) will result in it hitting
any given point for a shorter time than the larger spot from the Sun (which
usually doesn't move very quickly).

But, at least, perhaps you'll now have a bit more respect for that little HeNe
laser or laser pointer!

(From: Jim Webb (jim@glservices.org).)

The real problem behind this is that it is assumed that the power density is
the significant factor in the thermal damage mechanism. The ability of the
retina to dissipate heat is not dependent on the area covered, but the
periphery (circumference) of the exposed area! The blood vessels are in the
retina and not the sclera (the surface under the retina) - it is the blood
flow that dissipates the heat and so can only act on the *edge* not the middle
of the exposed area. In circumference terms, the ratio drops to 7 times.
Furthermore because the larger spot is less efficient at dissipating heat, the
effective power delivered by the laser beam is only about 2 times greater than
that of the spot formed by the sun.

At a distance of 1 mile (1,609 m), the beam from a typical helium-neon laser
(which is a quite well collimated source) will have spread to a diameter of
roughly 4 feet (48 inches, 1.3 m). However, it will still appear quite
bright. Why is this so?

(Portions of the following from: Don Klipstein (don@Misty.com).)

The fraction of light entering the eye for a large diameter beam is pupil area
divided by beam area.

Assuming a pupil diameter of 1/4 inch (6.3 mm, rather dilated but not fully
dark adapted which may approach 1 cm). The portion of the beam entering the
eye would then be the square of (1/4)/(48), which is about 27 millionths of
the total. Since the 4 foot diameter beam is not uniform but dimmer towards
the edges, I would say the eye could get about 35 millionths of the beam near
the center or 35 nanowatts (35 nW).

Note that close to the laser, the pupil size is going to be larger than the
beam diameter (which is typically less than 1 mm) and pupil size larger than
this will not affect the maximum possible power entering the eye (though it
will affect the probability of this occurring. (One suggested laser safety
practice is to brightly illuminate the laser lab to make your pupils smaller.
Even though there are times this will not reduce the severity of the worst
case, a smaller target reduces likelihood of this happening.)

However, where the beam diameter is equal to or larger than the pupil
diameter, the difference in pupil diameter between bright and dark adapted
eyes will be very significant - more than a 30-fold difference in power
entering the eye for this analysis.

I calculate that a 4 foot diameter 1 mW 632.8 nm beam appears about as bright
as a 100 W bulb does 88 feet away.

Although 35 nW is definitely eye-safe, it may look quite bright against pitch
black surroundings especially when the eye is fully dark adapted (the pupil is
wide open and the combined retinal/neural sensitivity is maximum as it is
after awhile when out at night) and may quickly result in a noticeable
afterimage. The effect is probably enhanced by the knowledge that the light
source is a laser and thus potentially damaging to your eyesight.

And, what would happen if the divergence of the laser in this example were
reduced by a factor of 10 so that the beam was only about 5 inches in
diameter? Then the laser at a distance of 1 mile would appear much much
brighter than a 100 W bulb less than 1 foot away! The reason it will be
much much brighter is that the laser will appear as a point source,
while the light bulb at 1 foot will be a large area. Imagine a pin-point
of light with same total optical power as the 100 W bulb.

As a side note, the 1,710 lumen output of a typical 100 Watt incandescent bulb
is about the same lumens as *10 Watts* of 632.8 nm light!

Since you likely did only receive the standard single (1) pair of eyeballs and
replacement isn't yet feasible (or covered by major health insurance plans!),
trying to figure out if your laser is a hazard to vision by staring into its
beam is a really really bad idea. Many factors can result in it being way to
late before you discover that your vision has been harmed.

Note that even a wavelength considered eye-safe like 1,500 nm (1.5 µm) is
only safe in the sense that this light won't penetrate to the back of the
eye and be focused on the retina. A high enough power density can still
obliterate the cornea and/or lens!

(From: Paul Mathews (optoeng@whidbey.com).)

There are a variety of problems with doing experiments to determine safe
levels of optical radiation incident on the eye. Here are some:

Subjects are generally not aware of any retinal damage until they notice
that parts of the visual field of one eye are blind. The visual system does
a good job of providing us with the illusion of perfect vision, in spite of
deficits. There is little or no pain in most instances.

Laser Safety Training by
Laser Professionals has a free Web program for calculating MPE,
laser safety eye-wear OD, and other safety parameters based on the
laser's characteristics. Click on "EASYHAZ". (Javascript must
be enabled.)

Safety Issues With Respect to Hobbyist Lasers

The most common types of lasers generally available to hobbyists - CD laser
diodes, visible laser diodes, laser pointers, and small HeNe lasers, are all
rated Class II or IIIa. See the section: Laser
Safety Classifications. Class II lasers should be relatively low risk if
even minimal precautions are taken. However, Class IIIa lasers must be taken
much more seriously if the beam is well collimated - as it would be from a
laser pointer or HeNe laser tube.

When you graduate to higher power lasers (e.g., argon ion) rated Class IIIb or
more, additional very real dangers are present of both instant damage to vision
and with Class IV lasers - the possibility of burning or setting fire to flesh
and other things. The smallest CO2 laser is going to be rated Class IV!

Higher power diode lasers (above 5 mW) are becoming more readily available both
as surplus or pulls from optical drives and high performance laser printers,
and also at not totally unreasonable prices even new. Their small size may
lead one to assume that a diode laser can't be dangerous. WRONG! A 100 mW
laser diode operating on battery power can blow a hole in your retina as
easily as a 100 mW argon ion laser consuming the same electrical power as a
space heater! And, higher power laser diodes are more likely to be infra-red
(IR) and invisible - and thus more dangerous because the aversion response
won't work - you have no idea your vision is being destroyed until it's way
too late! (CO2 lasers are also IR but the much longer wavelength will only
vaporize the front of your eye since the beam is blocked by the cornea.)

In addition to their vision hazards, gas lasers generally use high voltage or
line connected power supplies so there is the added shock hazard resulting
from touching or accidentally coming in contact with uninsulated connections.
See the document: Safety Guidelines for High Voltage
and/or Line Powered Equipment before working on any type of equipment
which uses line voltage or produces high voltage. (With diode lasers, you can
easily fry the laser diode but the low voltage power supplies don't generally
pose much of a shock hazard.)

Small HeNe lasers (say, under 5 mW) at least require low current (a few
mA) so the risk of actual electrocution from the a commercial high voltage
power supply is relatively small but there may be AC line voltage involved
and there can be collateral damage from a reflex response to the shock. But,
a homemade power supply may use components which are grossly oversized for
the application (due to low cost availability) like a 15,000 V, 400 W neon
sign transformer even though only under 10 W of power is actually needed (we
definitely do NOT recommend this approach). However, all power supplies for
larger HeNe lasers can be quite lethal.

Small Ar/Kr ion lasers operate at relatively modest voltage - 100 to 110
VDC across the tube - but due to the high current (up to 10 A), are usually
directly line connected (no line isolation) and therefore the power supplies
are extremely dangerous.

Small CO2 lasers do indeed use high voltage and possibly much higher
current than HeNe lasers - that neon sign transformer may be appropriate - and
deadly!

Note that some of these 'small' lasers are only small in comparison to their
higher power cousins and small doesn't equate to safe!

Furthermore, you may come across a truly high power CO2 or argon ion laser, or
even a 100 mW HeNe laser tube. These, rated at the upper end of Class IIIb or
Class IV, represent even more significant risks of both instant permanent eye
damage even from momentary reflections from shiny (specular) surfaces as well
an actual fire hazard. The possibility of electrocution from their power
supplies is correspondingly greater as well. You must handle them properly
for your own safety and the safety of others around you and your surroundings.

See the specific chapters on each of these types of lasers for additional
hazards and precautions Note that other people in the area may actually be
more likely to get caught by the beam. The reason? You will be aware of what
NOT to look at while they will be looking in the direction of the action not
having a clue of what to expect! Don't take chances.

The following very large number is designed to impress: The power density
of a 1 mW laser beam when focused to a spot of around 2 µm (which isn't
difficult with a simple convex lens) is around 250,000,000 W per square meter!
Don't let that spot be in the back of one of your own or someone else's
eyeballs!

Be extremely careful when working with any laser!

(From: Mike Poulton (tjpoulton@aol.com).)

A 1 mW diode will probably not cause damage if you briefly look into it, but I
wouldn't encourage you to try it. While it probably won't do anything bad, it
is not good to become comfortable with the idea of checking the operation of
lasers by looking into them. If you are a hobbyist who uses lasers quite a
bit, there is a good chance you will, at some point, end up with an unmarked
diode. It could emit any wavelength at any power level, and how bright the
beam appears when you shine it on something has no bearing on the power level.
Looking into an unmarked diode just because the beam is dim could (and
probably will) have disastrous results. I have a 1 W 808 nm laser diode, and
it appears much dimmer than a .5 mW 670nm beam when focused into a .2 mm spot.
When focused in that way, it will easily engrave plastic and burn paper and
wood (and skin). Just because it looks dim doesn't mean it won't instantly
blind you.

(From: Daniel P. B. Smith (dpbsmith@world.std.com).)

Be aware that eye damage that is localized to a small area of the eye is not
very noticeable. For example, few people ever notice the existence of the
large blind spot where the optic nerve enters the eye even though it is rather
huge (10 degrees or so) and not all that far from central vision. A laser
wouldn't necessarily have to make you totally blind; it could just wipe out a
teeny patch here and a teeny patch there. This kind of damage would be very
insidious; each time you'd say "Wow! That was bright! lucky I didn't get
blinded" - while slowly and cumulatively losing your sight...

These guidelines are for your own protection and that of others around you.
Lasers have a unique set of dangers not present with other equipment common at
work or at home. And, yes, some of these guidelines even apply to those $9.95
laser pointers!

Never look into the beam of any laser. OK, there might be exceptions if
you are *absolutely* sure the beam has been attenuated or diverged enough to
be totally eye-safe. For example, the beam from the optical pickup in a DVD
player is safe to view from an oblique angle at a distance of at least 6
inches since it is highly divergent; the beam from a supermarket barcode
scanner is safe because it is scanning rapidly; and the beam from a laser
rangefinder operating at 1.5 µm may be eye-safe if low enough power
density because it won't penetrate the cornea and lens of the eye.) Distance
alone isn't a guarantee - some lasers maintain a tightly collimated beams for
100s of feet or more. IR lasers may be invisible but can still cause instant
damage to vision and are even more dangerous than visible laser because your
blink and aversion reflexes don't work if you can't see the beam. Specular
reflections (from shiny surfaces like glass and metal) may be just as
dangerous as the raw beam. Viewing the reflection from a diffuse surface
like a white card is much safer though for higher power lasers, even if the
card doesn't burst into flames, the reflection may still be unbearably bright.

Wearing a set of proper laser safety goggles is a good idea when working
with any laser but especially for those rated Class IIIb or higher. Each
type of laser requires its own specific protection depending on wavelength
and power/energy. Just because you have a piece of colored glass or dark
visor from a welding outfit doesn't mean it will protect you from a laser
beam! Using eye-wear can even be important if you are working on a totally
eye-safe laser. Why? Because developing proper habits will mean that you
are automatically protected should you acquire a much higher power laser -
assuming you use the correct eye-wear!

(Portions from: Lynn Strickland (stricks760@earthlink.net).)

In addition to laser equipment and laser safety gear manufacturers, large
laser surplus outfits often have some minimal selection of laser safety
goggles, but those that are available will probably cover the types of lasers
you are using. However, they may not have all the regulatory approvals -
that's one of the things that boost prices! :) Also be careful whether the
eye wear is designed for diffuse viewing only, or will withstand a direct hit
from the laser. Know what you are getting - the worst thing is to think you
are protected when you are not. Or, to become so disgusted with the
reduction in visual acuity and clear view resulting from poorly made or
mismatched goggles that you end up not using them at all!

Be aware of the wavelength(s) power of your laser(s). A 100 W CO2 laser
and 100 mW Ar ion laser are quite different and require different sets of
precautions but one is not necessarily more dangerous than the other.
Specific laser classifications and precautions depend on both wavelength and
power.

Always terminate the laser beam with a light absorbing material or diffuse
screen. Don't just let it fly wildly around the room to end up
who-knows-where.

When adjusting or aligning a laser with the covers off, beware of reflections
from all optics surfaces. Those inside the laser cavity will have optical
power densities much higher than that of the output beam making even a small
percentage of reflection significant. For example, an argon ion laser
outputting a few hundred mW can have 10 or 20 mW reflected from each Brewster
window in two directions. These may be non-existent or weak when you start
out but can appear suddenly as adjusting screws are turned. The risks are
even more significant with a laser producing an invisible beam. Where
possible, put sleeves around the Brewster windows and block reflections
from other optics while the laser's innards are exposed.

Clearly mark the path of the beam and provide barriers to prevent
accidental contact with eyes (all lasers) and other body parts (high power
lasers).

Follow all relevant electrical safety regulations with respect to wire
sizes, equipment grounding, and proper hookup, as well as providing essential
fuses, circuit breakers, GFCIs, and other protection devices. Insulate or
block access to all AC line connected and/or high voltage terminals.

Provide a 'kill' switch in an accessible location away from the laser and
its beam path just in case you need to cut power in a hurry.

Put appropriate laser safety and electrical safety warning/danger stickers
near the laser emission aperture and other beam path locations, on the laser,
and on power supply components.

Never randomly aim a laser out the window. In fact, your laser lab or
workshop should have shades or blinds over all windows to prevent this from
happening by accident. Someone across the street may inadvertently look
into the beam. And, deliberately directing a laser toward an aircraft is not
only incredibly stupid but also highly illegal - pilots take their eyesight
quite seriously! There may be specific applications or experiments that
depend on using lasers outside (professional laser light shows, line-of-site
laser communications, surveying, LIDAR, etc.) but each will have its
additional specific safety precautions and regulations.

Instruct anyone else with you as to the hazards of laser light and make
sure they understand all of these guidelines. Those with you may actually be
in MORE danger because they will be looking toward the direction of the action
while you will know what to expect and avoid.

Also see the additional comments below, and the more specific information on
laser safety in each chapter for the specific laser(s) you will be using.

There have been some recent articles (mainly in the UK) about eye injuries
resulting from careless or malicious use of common laser pointers. In the
U.S., there have been numerous news reports which would lead the average person
to believe that the absolute end of civilization as we know it will result
from the proliferation of these devices. Although the potential for eye injury
is typically what comes to mind when one thinks of a laser, the possible side
effects - or collateral damage - that may result from aiming one at somebody
is at least as likely a cause for the current wave of hysteria.

Keep in mind that what gets reported in the popular press is not exactly
what you would call rigorously reviewed for scientific accuracy. And, if it
turns out that the outcome wasn't quite as reported originally, any correction
for a front page story is usually to be found in fine print buried on page 17!
Actual substantiated instances of long term or permanent effects on vision
resulting from momentary or unintentional exposure to a laser pointer's beam -
or even from prolonged intentional misuse - appear to be all but non-existent.
Flash blindness IS possible, but this is temporary and will clear up on its
own.

The above applies where the laser pointer has been manufactured and tested
to meet CDRH Class IIIa safely limits or below. Note that where these devices
originate from countries with less rigorous quality control or where an
internal current adjust pot can be twiddled or even if run at very cold
temperatures where laser diode output power is greater, to risk of eye
damage from intentional abuse, at least, may increase.

With respect to direct personal danger, potential damage to vision is the
only real consideration - there is no risk from radiation or enough power in a
beam of less than 5 mW to burn anything. However, from a public policy and
regulatory perspective, there are actually three areas of concern:

Flash blindness from momentary exposure or permanent damage to vision from
prolonged intentional misuse. Laser pointers are usually rated Class IIIa or
less which means that the power is low enough that the eye should be protected
from permanent damage by natural pupil contraction, blink, and aversion
reflexes.

Distraction and collateral damage - you wreck your car because someone
pointed a laser pointer at you while you were driving.

Misinterpretation of intent - you get blown away by someone with a BIG gun
who thinks you are targeting them with a laser sight. Or, you are arrested
and thrown in the slammer for aiming a laser pointer at a cop (this happened
recently).

I am in favor of tough laws to make (2) and (3) crimes and require at least
full restitution (maybe even 2X or 3X) for any resulting damages in addition
to disciplinary action or jail time. Such behavior should not be tolerated.
However, in the remainder of this section, I only really want to address the
vision issues (1).

While I absolutely agree that intentionally aiming a laser of any kind into
someone's eye is basically stupid (unless you are having laser eye surgery),
one must be careful in interpreting the meaning of press reports that describe
momentary exposure to the beam from a laser pointer waved around an auditorium
resulting in instant total loss of vision in all three eyes. One would have
to direct the beam into the pupil of the eye from a close distance for a few
seconds or more without either the eye or pointer moving, twitching, or
blinking. Distance is significant both because even laser pointer beams
diverge (especially cheap ones) so less energy is able to enter the pupil of
the eye as the source moves further away and it is harder/less likely for it
to remain stationary and centered on such a target a few mm across. This is
not really possible by accident and even takes significant effort to do
intentionally since the eye's natural pupil contraction, blink, and aversion
reflexes will prevent the beam from focusing on a single spot on the retina
with a sufficient concentration of energy for more than an instant - not
enough time for damage to result. There would have to be cooperation which
can only really happen in a game of chicken - but it is hard to protect people
from their own stupidity. This does mean, however, as if it isn't already
obvious, that laser pointers should be kept from infants - period, and away
from children unless adequately supervised. Adults, on the other hand,
presumably know not to stare into painfully bright lights and some may even
read the warning labels!

Though momentary exposure may indeed result in temporary flash blindness,
disorientation, multiple afterimages, and a headache, such effects,
while not to be minimized in importance, should not be permanent. And, as the
distance between the eye and the pointer increases, their severity and duration
diminishes greatly. To suggest any long term eye injury from a pointer's beam
originating on the other side of a football stadium is simply not plausible.

In fact, despite the great amount of press coverage lately - and such reports
resulting in the passage of laws in some places banning laser pointer sales to
minors (or to anyone), there are very few if any confirmed reports of permanent
vision damage attributable to these things. The irresponsible aiming of a
laser pointer at a person that might result in tragic consequences from
distraction or misinterpretation of intent is far more likely to be a problem
in today's world - and justifiably so.

Laser pointer manufacturers and resellers make all sorts of claims about power
levels and there may be deliberate (power is, after all, a major feature) or
unintended (due to poor quality control) sale of devices with power even beyond
the approved safety limits and these could indeed be much more dangerous.
However, simply enforcing existing regulations could go a long way toward
reducing this possibility. But, of course, the prices would likely go up if
more sophisticated laser power control circuitry were required and every unit
had to be more fully tested, adjusted, and certified to be compliant.

To further minimize the chance of vision damage, I think a maximum power limit
of 1 mW would be more than adequate for most purposes with the newer 635 nm
pointers. These appear 5 to 7 times brighter than previous 670 nm models and
green laser pointers which are now available at affordable prices - under $50 -
will appear even brighter by another factor of 5 or so.
Staring into the noonday Sun would result in the same order of magnitude of
power focused on the retina as a 1 mW laser pointer against your eyeball and
we don't even bother to regulate THAT! :)

Don't get me wrong - I am definitely NOT recommending that laser pointers be
treated as toys and handed out to all the neighborhood kids as party favors.
They can still be dangerous and at least a niusance even if eye injury isn't
the primary risk. I fully agree that any use of such a device in a way that
annoys other people or puts them at risk - even if it is a small risk - is
valid grounds for confiscation and possible severe disciplinary action.

For that matter, how come no one has banned butane lighters or matches? :-)
They are cheaper, more readily available, and certainly result in more injury,
death, and destruction in the hands of kids than laser pointers! Or, how about
cigarettes.... Sorry, I will get off my soap box now....... No, I don't
expect an answer. :-)

Note that at the same actual output power - say 5 mW since this is the legal
limit in the USA - there isn't all that much difference between red and
green laser pointers. Since the green wavelength of 532 nm appears much
brighter than even 635 nm red (the shortest wavelength from a red diode
laser (and most red pointers are closer to 650 nm), you'll be more likely
to look away faster with green than red. However, shorter wavelengths can
focus to a smaller spot producing higher power density and the receptors
in the eye may be more absorptive at the green wavelength.

However, if pointers are compared without regard to actual output power,
red pointers actually are incapable of producing much more than 5 mW no
matter how hard you try. They will just die if an attempt is made to
boost them much above 5 mW.

But most green pointers that use constant current drivers can produce much more
than 5 mW even if rated only 5 mW since turning up the current will increase
power - possibly substantially. This may even happen by accident or from poor
quality control at the factory - which is very common. Manufacturers are now
switching over to constant (optical) power drivers and adding means to prevent
tampering, so this will be less likely in the future.

The following is a report that deals specifically with legal
(5 mW) green laser pointers. (This is copyright by NEWSWIRE.)

Green Laser Pointer Can Cause Eye Damage

ROCHESTER, Minn., May 9 (AScribe Newswire) -- Mayo Clinic
ophthalmologists have found commercially available Class 3A green laser
pointers can cause visible harm to the eye's retina with exposures as short
as 60 seconds. The findings will be published in the May issue of
Archives of Ophthalmoloyg.

Dennis Robertson, M.D., Mayo Clinic ophthalmologist, conducted
investigations with a green laser pointer directed to the retina of a
patient's eye; the eye was scheduled for removal because of a malignancy.
The green laser damaged the pigment layer of the retina, although it did not
cause a measurable decrease in the visual function of the patient's eye. Dr.
Robertson believes that longer exposures could harm vision, however. He also
warns about potential damage from higher-powered green laser pointers.

"With the use of laser pointers that are more powerful than five
milliwatts, there would likely be damage to the actual vision," he says.
"Functional damage could occur within seconds."

Dr. Robertson does not advocate against use of green laser pointers;
rather, he advocates against their misuse. "Green laser pointers are not a
public health hazard at this time, but something people should be aware of,"
he says. "I'm raising concerns that people should be cautious when using
green laser pointers not to point them at someone's eye or face. It's like
how you use your knife -- carefully."

While pointing out risks of green laser pointers, he adds, "This is a
potential hazard to people's eyes, but rarely is it going to be a practical
hazard because the aversion reflex we have naturally will cause a person to
blink or turn away from a laser light."

Green laser pointers are readily available in stores and on the
Internet, according to Dr. Robertson. "Kids can buy these," he says.
"They're not strictly regulated."

He adds that Class 3A green laser pointers are increasingly being
used by amateur astronomers to pinpoint objects in the night sky and by the
construction industry and architecture educators to point out details of
structures in daylight.

Dr. Robertson conducted the eye exposure test with a consenting
patient two weeks before eye removal due to ring melanoma. The patient's
vision was 20/20, and the macular retina appeared healthy.

Dr. Robertson exposed the patient's retina to light from a
commercially available Class 3A green laser with an average power measured
at less than five milliwatts: 60 seconds to the fovea, the center of acute
vision; five minutes to a site 5 degrees below the fovea; and 15 minutes to
a site 5 degrees above the fovea.

Dr. Robertson had color photographs taken of the eye before and after
exposure to the laser.

Dr. Robertson examined the patient's eye 24 hours after laser
exposure. He found retinal damage characterized by yellowish discoloration
involving the pigment layer beneath the fovea and at the site of the
15-minute exposure above the fovea. Each of these sites developed a grainy
texture within six days. Study of the eye tissue under a microscope also
confirmed damage to the pigment layer in the laser-exposed regions.

Dr. Robertson has been interested in the effects of lights on the
human eye during his career, testing operating room microscopes, lights used
in the clinic, red laser pointers and now green laser pointers.

Previously, he determined red laser pointers to be quite safe. "I
tested different powers up to five milliwatts and could not create
recognizable damage in the human eye with the red laser pointers," he
explains. "So, at least a transient exposure to red laser pointers' light is
only of trivial concern."

Dr. Robertson attributes the risk differential between red and green
lasers to wavelength. "We know that the retina is infinitely more sensitive
to shorter wavelengths," he says. "The green lasers appear much brighter to
the human eye because of the shorter wavelength and can cause damage."

As chair of the ILDA (International Laser Display Association) laser safety
committee, I have been carefully following the thread on laser pointer safety
(in the sci.optics newsgroup - search via
Google Groups
for the complete saga). I have seen most of the articles
in the press on laser incidents/accidents in the UK. If you have a source of
factual evidence concerning these 'injuries', I would greatly appreciate the
information. My own experience with laser pointers would indicate that a
level of 5 milliwatts and below is unlikely to cause injury unless
self-inflicted and for a substantial duration (several seconds). I say
self-inflicted, as it is unlikely that another person could direct the laser
accurately into someone's eye at any significant range. Almost immediately
after the initial exposure to the beam, the pupil shrinks to a very small
size (a few millimeters) which is an awfully small target to illuminate from
a distance of even a few meters.

However, if there is any medical evidence of these injuries, and some
documentation of how they occurred (laser power, range, duration, etc.) I
am most interested.

The light source in a supermarket or other common barcode UPC (Universal
Product Code) scanner is either a .5 to 2 mW HeNe laser (632.8 nm orange-red)
or a 1 to 5 mW diode laser (most often around 670 nm, red). So, while the
beam may appear bright, as long as it is scanning at all, there is no risk to
vision or anything else. (The average power into your eyeball is probably
less than 10 microwatts.) And, as with laser pointers, you would really have
to go out of your way for there to be any possibility of damage even if the
beam was stationary due to a failure of the scanner.

You can tell the difference between the types of scanners by the color of the
light. The beam of the diode laser based scanners will appear a much deeper
red than that of a HeNe laser based unit. (If you are into lasers, this is
one of the 'rites of passage' so to speak - to check out the local groceries
and supermarkets!) Of course, the other way to tell is that if your store
installed checkout scanners when the UPC was new technology, and hasn't
upgraded since, they are almost certainly based on HeNe lasers. (Barcode
scanners of all types, shapes, and sizes are often available from surplus
outfits as well as on-line auctions like eBay. At the right price, they
represent an excellent source of laser and optics related parts - even if you
don't want to use the unit for their intended purpose.)

Laser hazards and laser safety classifications depend on wavelength but not
just because some colors are much more visible than others.

For wavelengths within the visible spectrum and near IR where the cornea, lens,
and vitreous of the eye are transparent, 1 mW is the same amount of power
whether it is near IR, red, or green. There will be slight differences in
damage threshold depending on wavelength (spot size on the retina, absorption)
but green is really not more dangerous than red, mW per mW for a beam that
reaches the back of the eye. Since green light at 555 nm *appears* about 30
times brighter than red light at 670 nm, the green laser may actually be
slightly less of a hazard since you will likely respond to it faster (and, in
the case of laser pointers in particular, a lower power unit may be adequate).

Beyond the visible - IR and UV - there are other issues. UV laser light, like
UV Sunlight can indeed have effects beyond just those due to the power density.
Fortunately, there aren't likely to be UV any laser pointers any time soon
even if there were a use for them (phosphorescent white boards?)! :-) Most
other UV lasers (excimer, helium-cadmium, frequency quadrupled YAG, etc.) are
not that common either (at least not that the typical hobbyist will acquire).
However, should you consider building the nitrogen laser (among the easiest
of home-built lasers), its output is at 337.1 nm which is near-UV
(UV-A range).

Near IR is perhaps the most dangerous since it progressively less visible the
longer the wavelength starting at about 1/250th visibility compared to 555 nm
and going down to 3E-14 visibility (estimated) at 1,064 nm. Yet, until well
beyond this (maybe 1,500 nm), the light can still pass through the anterior
structures of the eye to reach the retina and will focus reasonably sharply
despite not being visible. There will be no blink or aversion reflex so
damage can be done even for modest power lasers without any immediate
symptoms. Only later, will the pretty patterns engraved on your retina(s)
become evident (since your brain will initially tend to fill in and mask their
effects). And, they won't go away - ever!

At mid IR, the beam can still penetrate to the lens, heating it, which may
produce a cataract. Far far IR such as the 10.6 µm (10,600 nm) from a carbon
dioxide (CO2) laser is effectively absorbed and blocked by the cornea of the
eye - and it can be damaged in a similar way. And, almost all CO2 lasers
produce enough power (a few W to 10s of kW) that they are also hazardous with
respect to burning things (including other types of flesh) as well as actually
setting fires.

The long and short of it is that there is a threshold of laser power that will
be dangerous in various ways at ANY wavelength and no laser can be treated as
totally safe until the detailed specifications of the laser and its optical
system are known.

Many lasers generate outputs that are not the fundamental wavelength of the
lasing medium such as Nd:YVO4 or Nd:YAG at 1,064 nm. The most common is 532
nm such as produced in green laser pointers. A non-linear crystal inside
the laser cavity uses non-linear process called Second Harmonic Generation
(SHG) to double the 1,064 nm to 532 nm. Other (usually scientific or
industrial) lasers may use Third Harmonic Generation (THG) or Four Harmonic
Generation (FHG), or some other process like sum or difference frequency
mixing to produce other wavelengths.

Unless your laser is set up for harmonic generation, there will be no higher
frequency radiation in the beam. The only accidental source of harmonics that
could pose a risk would be for the beam from a high peak power pulsed laser
(most likely it would need to be Q-switched) to pass through a non-linear
material that has good optical quality AND for the reflection from some surface
downstream to hit you in the eye. Materials with both those properties
do not occur naturally. Aiming the beam at common plastics, glasses, and
crystals won't produce significant, if any, harmonics. Finding a household
material that does so could be an important discovery. :)

To be doubly sure, you can buy goggles that protect for both the fundamental
and doubled outputs (e.g., 1,064 and 532 nm, harmonics above SHG would be
virtually impossible.) But they will be darker than those for the fundamental
along and thus less desirable. They are also more expensive. If you intend
to experiment with SHG, etc., or acquire a such a laser, that could be a
worthwhile investment.

A great deal of laser safety is in good work practices (outlined elsewhere in
this chapter). Goggles are just the last resort when everything else goes
wrong. However, for pulsed lasers where the entire beam path isn't enclosed,
they really are essential.

Fluorescence is a process whereby a high energy photon is absorbed by a
material which then emits a photon at a lower energy. For example, aiming
a green laser at a DayGlow(tm) sign or often those bright orange Fragile
stickers on packages will result in a bright yellow glow at the point where
the beam hits. The intensity may be 25 to 50 percent or more of the incident
beam. However, fluorescence phenomena do not produce a beam, only a diffuse
glow so there is generally no risk of eye injury from reflections of the
fluorescence or even direct viewing unless the laser is extremely powerful
(e.g., several watts or more).

While thumbing through some gel filter sample packs, it has occurred to me that
there are neutral density gel filters - and that they are not truly neutral.
Both Gam and Rosco ones are somewhat neutral through to about 700 nm - and
become more transparent as wavelength increases through the low and mid 700's.
They are nearly transparant above about 750 nm.

They also have a slight peak at 380 nm, where they are a bit more transparent
than they are to visible light. Transmission at 380 can exceed the average
visible transmission for darker grays.

This is because these filters are made gray with some kludge of dyes rather
than something truly neutral-density. They also do not equally attenuate all
visible wavelengths; they have transmission peaks around 480 (greenish blue)
and 600 (orange), and absorption peaks around 450 (mid-blue) and the mid 500's
(yellowish green). Different brands may have some differences, as well as
having some similarities. They probably have some but not all dyes in common.

I do not know whether the infrared transparency is an unavoidable consequence
of dying plastics/gels, or something intentional to reduce filter heating. I
do know that the colored filter gels are also nearly transparent to most
wavelengths from the upper 700's (sometimes low 700's) through probably at
least around 1500 nm.

Because of this, dark filter gel combinations are probably unsafe for directly
viewing the sun, and are probably unsafe for attempting to protect eyes from
infrared lasers.

I realize that no matter what is said, many people will not want to invest in
laser safety goggles. OK, so be it. However, there are things you can do
to minimize the chance of eye damage when working with Class IIIb lasers
at least. (For Class IV lasers, you're on your own!) In either case, we
won't be responsible for the consequences!

Much, if not most, of being safe around lasers has to do with work habits.
Laser safety goggles are only protection of last resort.

Fully enclose the beam path of your laser wherever it is reasonably well
collimated in such a way that it is impossible for anyone's eyes to intercept
the beam. The enclosure can be transparent (e.g., Plexiglas), a wire mesh,
or something else as long your eyeballs are kept out. The beam must also be
safely terminated. A collimated beam is the most dangerous since it can be
focused to a microscopic spot on the retina. A highly divergent beam - even
a high power one - is much less of a hazard unless something is very close to
the source.

Put beam stops whereever there may be stray reflections (as from
optics along the beam path, even is AR coated), especially important if
they leave the plane of the setup.

Always locate lasers and all the beam paths well below eye level. (Above
eye level is also acceptable but I can't imagine it being convenient!) So,
you can't work sitting down unless the lasers are on a kiddy-height table
or you're on a bar stool! This puts your eyes above the area of danger.
Sorry. I know it's bad for backs but backs heal, retinas don't. Where
possible, work on the side of the laser beam's path, not in front or behind
it for similar reasons.

With adjustable lasers or where an attenuator is present, run at reduced
beam power for as much testing as possible.

Where none of these are possible - as with using green laser pointers to
identify astronomical objects in the sky, all I can suggest is to
take as many precautions as possible. Only use a pointer that has the normal
momentary switch so it will go off instantly if dropped and make sure all
the observers are aware of the dangers of Class IIIb lasers so they won't
do anything stupid. Even a momentary exposure at the higher power levels
often used in these activities - especially to dark adapted eyes - can
result in permanent eye damage.

The safest way to view the beam or objects illuminated by a visible to
near-IR laser is indirectly using a video camera (e.g., Web cam or
camcorder) and monitor. There is no way the laser beam will sneak
through the lens of the camera to hit you in the eye except by
reflection! Modern video cameras using CCD or CMOS image sensors have
a response from deep violet (and maybe near-UV) to near-IR out past
1,100 nm. This covers most of the lasers of interest to the
experimenter and hobbyist except the CO2 laser. However, it will
probably be necessary to remove a built-in IR blocking filter to get
decent IR response. Note that the appearance on a color video monitor
of IR well beyond the red-end of the spectrum - say 800 nm - will
likely be white or even blue-white, not red as might be expected.
This may be because the color filters used in the image sensor are
dichroic coatings optimized for the visible spectrum and all three
(RGB) have high transmittance in the IR.

I have one of those $50 video cameras that are sold by various electronics
distributors. This particular one is listed for IR and comes with 4 IR LEDs
(IREDs) for illumination (which I removed). It works fine except that there
is no way to defeat the automatic gain control so it gets confused with very
bright sources like lasers. I have also been given a very nice digitally
controlled color CCD camera. This has a Windows interface and provides full
control of gain, offset, and other parameters. For low power lasers, this
can be used without a lens viewing the beam diractly. Where there is a
risk of damage to the CCD, the beam is projected on a screen.

(From: Dave (ws407c@aol.com).)

I use a CCD video camera with the proper filters in line to balance the
sensitivity to the laser lines (e.g., 808 nm pump, 1,064 nm IR beam, 532 nm
green beam) and mount this outside of a cardboard box(helmet) with a 9" LCD
flat-panel display mounted on the inside. With this contraption over my head
I can see everything clearly and without any worry of eye damage and have
both hands free. My first version was my autofocus digital camera in a box
but the screen was too small for long duration work.

I would have to say that proper eye protection is much more important than any
laser component. This cannot be stressed enough. There have been some
interesting demonstrations performed showing the effects of high optical power
densities on meat (think lots of smoke and some flame). The ones I've seen
on videotape were spectacular, and were more than enough to convince me that
proper beam blocks and eye/body protection are mandatory.

When in doubt, be overly safe.

You should have enough pairs of laser goggles for everyone in your laser lab!
After all, what's the fun of a laser if you can't show it off to your friends?
;)

Now, about the ratings of goggles in terms of optical density.

Optical densities are reasonably easy to understand. To determine the
fraction of optical power transmitted through a material of optical density D,
divide 1 by 10 raised to the D power. Or, if D is an integer, just write a
zero followed by a decimal point, followed by D-1 more zeros, followed by a 1.
This is the fractional transmittance of the material. Multiply by 100 if you
want a percentage.

For our O.D. 5+ goggles above, this means that less than 1/100,000 of the
incident power will pass through the goggles, the remainder either being
reflected or absorbed. For a 100 Watt laser with a 1 square centimeter beam
(power density 100 W/cm2), the transmitted power density should be
0.001 W/cm2 (or 1 milliwatt per square centimeter). I have to
locate my safety sheets to see what the exposure limit is for eyes and skin
under a 1 mW/cm2 beam at 10.6 microns. I suspect it is eye and
skin safe, but without a good reference, I'm not betting my body parts on
it. ;)

Keep in mind that the O.D. is rated AT A SPECIFIC WAVELENGTH OR RANGE OF
WAVELENGTHS! Deviations from this wavelength will results in completely
different O.D. values. If the goggles use some type of interference coatings,
then at some wavelengths the coatings may have an effective O.D. of ZERO,
meaning they are completely transmissive. Don't expect CO2 goggles to protect
you from an argon laser beam.

By now, the following should be intuitively obvious but it never hurts to
retate it. While the use of laser safety goggles is highly recommended in
most situations when dealing with lasers, it is possible for them to do more
harm than good. This would be the case:

When they are not the correct type: Laser safety eye-wear that use
band blocking filters will only be good for a particular narrow range of
wavelengths. A set designed for Nd:YAG at 1,064 nm probably won't do anything
useful for ruby at 694 nm except provide some protection from an exploding
flashlamp!

When they are too good: If the attenuation so high at the laser
wavelength that essentially nothing gets through, you won't be able to
make adjustments that require some visibility of where the beam lands.
The best are probably goggles that attenuate the laser only enough to be
safe, not 100 percent. For example, OD4 for a 1 W laser so the maximum
transmitted power is 100 uW or less. You wouldn't want to stare into that
beam but it or a reflection will be very visible if you do so by accident.
Or, if they make everything too dim to see what you are doing - period.
Newer goggles and higher performance (and probably higher priced) googles
are better in this regard with more selective coatings or dyes. Pay
attention to the specifications. Welders' goggles are not the solution!

When you peek around them or take them off to see what you are
doing: Ease them off slowly! That way, scatter will clue you in to the
beam location, especially if it is next to your eyeball!

When they make you too complacent about the dangers of your laser:
Laser eye-wear won't protect you from the high voltage. :) Or, from damage to
other parts of your anatomy from a Class IV laser.

When only you are wearing a pair and you have visitors: You may
tend to do things that would be reckless without goggles but others in the
vicinity won't know what to avoid.

Realistically, if all you will ever be working with are visible lasers
of Class II or less, the use of laser safety goggles may be excessive.
However, by wearing goggles and treating even that low power beam with
respect, you will develop habits that would help to protect you (given the
conditions, above) should you graduate to higher power lasers. Just as the
recommendation in some laser safety classes to treat every laser beam - even
one from a laser pointers - like it will slice cleanly through you and never
let a laser beam intersect with any part of your anatomy (see the next section
and the one that follows), making laser safety eye-wear part of your routine
can be a vision saver when dealing with a 100 W YAG instead of 1 mW HeNe!

There is a nice article in the March, 2005 issue of
Photonics Spectra describing
5 incidents where carelessness around high power lasers, some of which
resulted in permanent serious vision loss to scientists who should have
known better.

(From: Richard Alexander (pooua@aol.com).)

During the 1st Trimester of the laser program, long before the student is
allowed near so much as a HeNe laser, the students are shown "the monkey
film." This is one of the films that would drive PETA nuts. All you see is
this eyeball, which we are told belongs to a monkey that is strapped down and
anesthetized. We are told that an IR beam of a certain power has been turned
on, and is striking the eyeball. After an eternity (a second or 2), a small
spot appears on the bottom of the eyeball. Then, the spot rapidly expands,
forming an ulcer that covers much of the bottom of the monkey's eye. I don't
know if that was in slow motion or not.

We also read accident reports from the field. There was a technician who was
working on an extremely powerful laser. He had removed his eye protection, and
walked across the room. There was a weak stray specular reflection that struck
one of his eyes, immediately causing permanent damage to that eye. He did not
lose all of his sight in the eye, but he did lose part of his field of vision.

From the first time that the laser students operate a HeNe laser, they are
required to treat the beam as lethal. Under no circumstances are they
permitted to break any beam of any power with any part of their body. Our
little HeNe beams could not cause damage to skin, but we had to act as if they
would cut off our arms. The student is also responsible to ensure that he
knows where all the parts of the beam go, and to block the beam appropriately.

The HeNe laser labs had curtains across the doorways, which we closed before
beginning experiments. The Argon Ion and Nd:YAG labs had solid doors, and
there were sensors in the doors that would cut off the power to the lasers if
the door were opened. There was also a red warning light that was to be turned
on when the laser was in operation.

Normally in the laser show world, you deal with eye injury, lasers up to about
5 watts or so, typically only a few hundred mW though. A few hundred mW on
your skin simply "looks cool", while 30 watts will quickly blow a hole right
through your whole hand and out the other side! The worst thing about this is
it is absolutely painless. Not black burned skin, but white ablated skin.
Blows a hole right through, not even smoke is left behind. You don't feel
the pain from such an injury for many minutes AFTER, then it's excruciating!
I won't even go into what would happen if you took 30 watts into the eye
directly. Also, most of us have seen bright laser spots on white surfaces
(projection screens) up close, and know how "blindingly bright" they are, but
also that the plain-air beams are invisible. Imagine a laser where the
plain-air beams hurt the eye to look at! :) The spot on a surface is so
bright as to light a room up as though it was bright sunlight (in green) given
that the spot was expanded to around 30 mm so as not to burn a hole in the
surface.

Several years ago there was a long thread on the USENET newsgroup rec.guns
where people posted their stories about all the accidents or near accidents
they had experienced with firearms. These were all seemingly intelligent
people like computer programmers and scientists and engineers. Still, while
dealing with a simple device with only a few knobs, they managed somehow,
sooner or later, and while trying to obey all the safety rules, to blast a
hole in something or someone. This was very educational reading.

There was a really good story recently posted on sci.optics. Some guy
was working with a laser, and then took off his goggles blowing out some
of his eye. Dumb. Then, rather than realizing that the goggles don't work if
they are not worn, he decided that he just wouldn't wear them at all, and he
would Be Real Careful. This is called "People Who Don't Learn From Their
Mistakes". Let's hope he doesn't take up firearms.

When you have goggles on (assuming that they are the right kind, and you
should make damn sure they are), you have very good protection against loss of
vision. When you take them off, you don't. A movement of a mirror, lens, or
baffle can cause a specular reflection, total internal reflection, or
refraction right into your eye. This isn't something to anticipate -- that's
why it is called an accident. Even with all the appropriate precautions,
accidents can still happen.

Imagine that you are working with the laser off, aligning some mirror, no
goggles, and you spill your coffee over the on-off switch to the laser power.
Oops. Collect insurance.

This is the first draft of a section on this topic. I welcome comments
and additions/corrections.

If you are a laser user, there is only one rule: Under no circumstances
should ANY laser be pointed in a direction that might intercept any
aircraft. Period. For research purposes and laser shows, there will
be specific protocols to follow such that any laser beams shot skyward
will not come anywhere near planes.

If you are a pilot, the recent news reports of incidents supposedly involving
lasers pointed at commercial airplanes from the ground must be of concern.
But how to sort the facts from the hype and exaggerations?

For the following, a fixed wing airplane is assumed. Helicopters, balloons,
and other types of aircraft that can hover or travel slowly do make more
inviting targets since the aiming is easier and the beam could be maintained in
the cockpit area longer. However, most of the Press has been with respect
to commercial airplanes - so far. And, the hover or slow movement works
both ways - they can more easily spot the origin of any laser beams and
report the location to the authorities.

For the time being, only continuous wave (CW) lasers will be addressed. These
are by far the type most likely to be involved in these incidents. Some
green laser pointers are quasi-CW - they typically produce a beam that's
chopped at a rate from 500 to 5,000 Hz - but don't generate the high peak
power of true pulsed lasers. Thus, the information still applies to them.

There are many variables to consider when separating fact from fiction.
These include:

Laser wavelength: This is the first thing that needs to be
considered. For the purposes of this discussion, it can be divided
into several ranges: UV, visible, near-IR, and far-IR.

UV (below approximately 400 nm): The availability of UV lasers,
especially above a few mW of output power, is very limited. While some
relatively high power (greater than 100 mW) UV lasers do exist, they
are large, power hungry, expensive, and generally limited to research
labs, semiconductor fabs, and LASIK eye correction clinics. The
probability of anyone attempting to use such a laser to target a plane
is extremely small. None of these are even remotely portable.

Visible (400 nm to 800 nm): This includes the spectral colors from
violet through red. While textbooks usually quote the range as being
from 400 to 700 nm, most people can still perceive something to well
beyond 800 nm, some to beyond 900 nm. But the sensitivity is so low
that very high power is needed to evoke even a moderate perceived
brightness.

Visible lasers come in all colors. But by far the most likely ones to
be used to harass airplanes are red and green - from inexpensive laser
pointers. Why? Because the most likely culprits are likely to be
stupid kids with nothing better to do who have received laser pointers
as gifts. Although some Press reports have involved supposed incidents
involving high power lasers and extended duration tracking, most of these
are rather suspect and impossible to verify. The other likely sources
are errant beams from laser shows or advertising extravaganzas that were
somehow not properly regulated.

All pointers are legally limited to 5 mW. The laser diodes used in red
pointers are simply not capable of producing an output power much above
5 mW without failing permanently.

Legal green pointers are also limited to 5 mW. Until recently, green
pointers were very expensive ($300 was typical only two or three years
ago) and thus not nearly as common as red pointers, which can sometimes
be obtained for literally $1. But, within the past year or so, prices
have plummeted to below $50 making them much more widely owned. So, it's
not surprising that aviation incidents using green
pointers have increased. Furthermore, because of the Diode Pumped Solid
State (DPSS) laser technology that is used, it has been very easy
to significantly increase output power on many models of green pointers
to way above the legal limit with simple modifications. In fact, this
has been known to happen by accident and some stock green pointers will
produce more than 5 mW just due to power fluctuations that occur as they
warm up.

Incidents with green pointers are also likely to be more obvious because
the perceived brightness of the green (532 nm) wavelength compared to
red (635 to 680 nm) is 4 to 15 times greater. Because of this, the green
wavelength is also more likely to be distracting. However, for this
reason, they are also less likely to result in permanent injury as the
aversion/blink reflex is more sensitive.

Because some green pointers can be boosted in output power relatively easily
to 50 mW or more (and are available at various Web sites already running
on steroids), there is the potential for more serious incidents, though
actual permanent damage to vision, or even flash blindness, is extremely
unlikely at the altitudes and speeds of fixed wing airplanes.

However, many other types of visible lasers are readily available surplus,
from eBay, and elsewhere. While these can go to very high power (WATTs),
again, it's a matter of cost, size, weight, power requirements.

Near-IR (750 nm to 3 µm). Most of these lasers will be either high
power laser diodes operating at wavelengths between 790 and 990 nm, or
solid state lasers almost exclusively operating at 1,064 nm. While
some longer wavelength near-IR lasers exist, they are not at all common.

While it's possible to target a plane with a high power IR laser, this
would require a level of expertise to construct as there are no common
lasers out there which could be used without modification, and no hand-held
ones at all.

Far-IR (beyond approximately 3 µm): The only common laser operating
in this range is the carbon dioxide laser at 9.6 to 10.6 µm. While these
are high power (10 watts and up) and surplus CO2 lasers are readily
available, the 10.6 µm wavelength does not penetrate glass or plastic
so unless you're flying a WW-I open cockpit biplane, the cockpit windows
will be effective protection. At high enough power levels, the windows
could be destroyed but that will only happen at power levels available
from classified Government laser weapons - in the kilowatt range, requiring
a large truck to transport and provide power.

Power of the laser: A high power laser will obviously be more of a
threat than a low power one, but divergence, distance to the plane, and
all the other factors are at least as important.

Divergence of the beam: A larger divergence makes it easier to aim
and maintain contact but reduces the power density. It's possible to
reduce the divergence of most lasers using simple optics - one half of
a binocular in reverse will decrease the divergence by the magnification
factor (e.g., 7x50 would reduce it to 1/7th of its original divergence).

CW, quasi-CW, or pulsed laser: For now, we are only considering CW
lasers since these are the most likely types to be involved in these
incidents. Some pointers are quasi-CW but don't have the high peak power
of pulsed lasers, so the information will be valid for them.

Distance to the aircraft: This affects the spot size based on the
divergence, and the ability to aim the laser.

Angle above the horizon: This limits the distance at which a laser
beam can actually make its way into the cockpit unless in a turn. If
too close to the plane, the angle will be too steep.

There are three types of effects that need to be considered:

Distraction: This is by far the most likely result of a laser beam
entering the cockpit of an aircraft, particularly at night. Dark adapted
vision is critical to the safe controlled operation of an airplane, especially
during takeoff and landing maneuvers. Any distraction during final approach
or in a steep turn could have disastrous consequences. An unexpected
momentary flash from even a legal (<5 mW) laser pointer 1,000 feet
away could be enough to cause disorientation. It doesn't take a fancy
laser to have the potential for distraction.

An example: A typical 5 mW green laser pointer has a beam diameter of 1
millimeter and a divergence of about 1 milliradian - the beam expands
at a rate of 1 part in 1,000. So, if aimed at a plane from the ground
during final approach from 1,000 feet away, the beam would be about 1
foot in diameter (1/1,000 times 1,000 feet). The power density of the
resulting 1 foot spot would be about 8 microwatts per square centimeter.
This is well above the power that is considered to be distracting to
dark adapted eyes. It's around the same brightness as a 100 W light bulb
at 10 feet - which would appear dazzlingly bright if occurring as a flash
in near-total darkness.

However, it's orders of magnitude away from being capable of causing
permanent eye injury or even afterimages.

Flash blindness: Above a power density of perhaps a few dozen
uW/cm2, there will be a reduction in visual sensitivity
and possibly afterimages for seconds to many minutes after the event. This is
a temporary condition but unless there is another unaffected pilot
to fly the plane for those few minutes, it could ruin your whole day.

Permanent eye damage: Above a few 10s of mW for a momentary flash,
or 5 or 10 mW for a sustained beam, permanent irreversible effects are
possible. These result in lesions on the retina visible with suitable
eye examination techniques including something called a fluorecein
angiogram. The visual effects may include distortion, wavyness, as well
as holes in the visual field. However, note that unless this happens in
the central (foveal) area of the retina, the effects may not be detected
immediately as the brain is quite good at filling in missing information.
Once damage like this occurs, complete recovery of the affected areas
is unlikely, though some improvement may take place over the course of
weeks or months.

Protection for pilots:

Since the majority of incidents are likely to involve green lasers, and
green laser pointers specifically, having goggles available with a narrow
band filter response providing high attenuation at 532 nm would provide
excellent protection with virtually no effect on color vision. It should
also be possible to add filtering for IR and UV wavelengths with minimal
effect on percent of light transmission. Such goggles would protect both
from annoying laser flashes as well as much less likely higher power
lasers that have the potential for permanent injury.

While this doesn't address other visible wavelengths, the most common ones
would be various red wavelengths from 630 to 680 nm. Laser pointers in
this range of wavelengths are limited to 5 mW by law, but more importantly,
not much more than this by the technology. Unlike green pointers that
can relatively easily be boosted in power, sometimes by just turning
a pot inside, the laser diodes in red pointers just die if pushed
much above 5 mW. Although it's possible to replace the 5 mW diodes with
higher power ones - up to more than 100 mW are now available - this requires
a significant level of skill. While medium power red lasers are available,
they are not common and most are not portable.

As has been noted above, protection for far-IR (beyond a few µm) is
provided automatically by the glass or plastic of the cockpit windows.

Probabilities:

The important thing is not to become paranoid. Figure the odds: How many
takeoffs and landings are there in the USA, the World, each day? How many
incidents have been reported? Why would someone want to target your plane?
If that's not enough reassurance, get yourself a set of of high quality
multiwavelength laser safety goggles as noted above. It's a small investment.
You don't have to wear the goggles all the time, just keep them at hand - it's
very likely there would be some warning of someone attempting to target your
plane. The googles should deal with 99.9 percent of the lasers likely to be
used.

Sure, you can worry about high power IR lasers or laser weapons. It's
more likely you will suck a goose into the engine intake. You can also get
squashed by a bus walking across the street, and that's really a lot more
likely than getting hit in the eye by a laser while flying!

(From: L. Michael Roberts (newsmail@LaserFX.com).)

Patrick Murphy, who was very involved in the discussions which led to the
current regulations regarding outdoor laser shows, posted the following:

The FAA study addresses points including whether having a brief flash-like
exposure is not harmful compared with a full-on, steady illumination. Also,
whether pilots really can be temporarily flash-blinded by light levels 10 to
100 times below what we think of as flash-blinding levels.

For those who want to skip the details, here's a summary:

Even a single, 1-second-long exposure at a very low light level
(one-half microwatt per cm2.) can have a moderately negative effect
on pilots' ability to operate the aircraft during final approach. At
this level, which is 5000 times lower than the Class IIIa limit of
2.5 mW per cm2, 18% of the pilots felt they had been flash-blinded,
and 13% reported afterimages. At ten times this level, a still-low 5
microwatts per cm2, 21% of the pilots had actual or potential
aborted landings.

The study is not perfect but it is the best data available. It
demonstrates that pilots do feel their landings are disrupted by light
levels that laserists would normally think of as acceptable because
they are so short (just 1 second) and so low (50 to 5000 times lower
than Class IIIa exposure limits). The subjective experience of
laserists, that a particular light level is reasonable, may not be the
same for all people -- and especially not for pilots who are
concentrating on landing a commercial airliner.

Laser Safety Classifications

There are ANSI, OSHA, FDA (CDRH), NRPB, and military standards. The CDRH
(Center for Devices and Radiological Health is part of the Food and Drug
Administration and is the most relevant regulatory organization in the USA
for commercial and scientific lasers. The complete CDRH document may be found
at:
Performance Standard for Light Emitting Products.

As of Summer 2007, there is an updated "ANSI Z136.1 (2007) Safe Use of Lasers"
which among other things substitutes Class 3R for Class 3A, add Classes 1M
and 2M, changes some of the control measures and terminology, and more.
Nothing earth shattering though. I have not yet seen a full copy. However,
there is a summary article in the June 2007 Photonics Spectra magazine.
And a narrated slide show of the changes can be found in the
Laser Institute
of America ANSI Z.136.1 Presentation. However, without details or access
to the full document, it's an excellent cure for insomnia at best. :)

The best discussion of the various classifications, plus general treatment of
the topic, is a book by Sliney and Wolbarsht, "Safety with Lasers and Other
Optical Sources", Plenum Press, New York. While they will agree with each
other in most respects, some differences will result in a particular laser
changing classes depending on which standard is used. The major criteria
are summarized below.

Note: I may use Class 1 and Class I, Class 2 and Class II, Class 3 and
Class III, and Class 4 and Class IV interchangeably. They are equivalent.

The following is based on material from the University of Waterloo - Laser
Safety Manual.

All lasers are classified by the manufacturer and labelled with the
appropriate warning labels. Any modification of an existing laser or an
unclassified laser must be classified by the Laser Safety Officer prior to
use. The following criteria are used to classify lasers:

Wavelength. If the laser is designed to emit multiple wavelengths the
classification is based on the most hazardous wavelength.

For continuous wave (CW) or repetitively pulsed lasers the average power
output (Watts) and limiting exposure time inherent in the design are
considered.

Lasers are generally classified and controlled according to the following
criteria:

Class I lasers - Lasers that are not hazardous for continuous
viewing or are designed in such a way that prevent human access to laser
radiation. These consist of low power lasers or higher power embedded lasers
(i.e., laser printers).

Class II visible lasers (400 to 700 nm) - Lasers emitting visible
light which because of normal human aversion responses, do not normally
present a hazard, but would if viewed directly for extended periods of time.
This is like many conventional high intensity light sources.

Class IIa visible lasers (400 to 700 nm) - Lasers emitting visible
light not intended for viewing, and under normal operating conditions would
not produce a injury to the eye if viewed directly for less than 1,000
seconds (i.e. bar code scanners).

Class IIIa lasers - Lasers that normally would not cause injury to
the eye if viewed momentarily but would present a hazard if viewed using
collecting optics (fibre optics loupe or telescope).

Class IIIb lasers - Lasers that present an eye and skin hazard if
viewed directly. This includes both intrabeam viewing and specular
reflections. Class IIIb lasers do not produce a hazardous diffuse reflection
except when viewed at close proximity.

Class IV lasers - Lasers that present an eye hazard from direct,
specular and diffuse reflections. In addition such lasers may be fire
hazards and produce skin burns.

Here is another description, paraphrased from the CORD course: "Intro to
Lasers". (Cord Communications.
Lasers.) It relates the laser classifications to common laser types
and power levels:

Maximum power less than 0.4 uW for long term exposure (greater than 10,000
seconds). Looking at a Class I laser will not cause eye damage even where
the entire beam enters the eye and it is being stared at continuously.

A laser may also be labeled as Class I if it is entirely enclosed and not
accessible without disassembly using tools. Thus, a DVD burner with a
150 mW laser diode (normally a Class IIIB laser) would still be considered
Class I.

The classifications depend on the wavelength of the light as well and as noted,
there may be additional considerations for each class depending on which agency
is making the rules. For example, the NRPB (British) adds a requirement for
Class IIIa that the power density for a visible laser not exceed
25 W/m2 which would thus bump some laser pointers with tightly
focused beams from Class IIIa to Class IIIb. For more information on laser
pointer safety and the NRPB classifications, see the
NRPB Laser Pointer Article.

Here are some excerpts from the Center for Devices and Radiological Health
(CDRH) regulation 21 CFR 1040.10 and 21 CFR 1040.11, the standard
classification for lasers are as follows with some additional comments by
Wes Ellison (erl@sunflower.com):

Class I laser products

No known biological hazard. The light is shielded from any possible viewing
by a person and the laser system is interlocked to prevent the laser from
being on when exposed. (large laser printers such as the DEC LPS-40 had a
10 mW HeNe laser driving it which is a Class IIIb laser, but the printer is
interlocked so as to prevent any contact with the exposed laser beam, hence
the device produces no known biological hazard, even though the actual laser
is Class IIIb. This would also apply to CD/DVD/Blu-ray players and recorders
(which might have Class IIIb laser diodes of 100 mW or more) and small laser,
as they are Class I devices).

Class II laser products

Power up to 1 milliwatt. These lasers are not considered an optically
dangerous device as the eye reflex will prevent any occular
damage. (I.e., when the eye is hit with a bright light, the eye lid will
automatically blink or the person will turn their head so as to remove the
bright light. This is called the reflex action or time. Class II lasers
won't cause eye damage in this time period. Still, one wouldn't want to look
at it for an extended period of time.) Caution labels (yellow) should be
placed on the laser equipment. No known skin exposure hazard exist and no
fire hazard exist.

Class IIIa laser products

Power output between 1 milliwatt and 5 milliwatt. These lasers can produce
spot blindness under the right conditions and other possible eye
injuries. Products that have a Class IIIa laser should have a laser emission
indicator to tell when the laser is in operation. They should also have a
Danger label and output aperture label attached to the laser and/or
equipment. A key operated power switch SHOULD be used to prevent unauthorized
use. No known skin hazard of fire hazard exist.

Class IIIb laser products

Power output from 5 milliwatts to 500 milliwatts. These lasers are
considered a definite eye hazard, particularly at the higher power levels,
which WILL cause eye damage. These lasers MUST have a key switch to prevent
unauthorized use, a laser emission indicator, a 3 to 5 second time delay
after power is applied to allow the operator to move away from the beam path,
and a mechanical shutter to turn the beam off during use. Skin may be burned
at the higher levels of power output as well as the flash point of some
materials which could catch fire. (I have seen 250 mW argons set a piece of
red paper on fire in less than 2 seconds exposure time!) A red DANGER label
and aperture label MUST be affixed to the laser.

Class IV laser products

Power output >500 milliwatts. These CAN and WILL cause eye damage. The Class
IV range CAN and WILL cause materials to burn on contact as well as skin and
clothing to burn. These laser systems MUST have:

A key lockout switch to prevent unauthorized use Inter-locks to prevent the
system from being used with the protective covers off, emission indicators to
show that the laser is in use, mechanical shutters to block the beam, and red
DANGER labels and aperture labels affixed to the laser.

The reflected beam should be considered as dangerous as the primary
beam. (Again, I have seen a 1,000 watt CO2 laser blast a hole through a
piece of steel, so imagine what it would do to your eye !)

Registration of laser systems

Any laser system that has a power output of greater than 5 milliwatts MUST
be registered with the FDA and Center for Devices and Radiological Health if
it has an exposed beam, such as for entertainment (I.E. Laser light shows)
or for medical use (such as surgery) where someone other than the operator
may come in contact with it. (This is called a 'variance' and I have filled
them out and submitted them and they ARE a royal pain in the backside!)

Sometimes, you will come across a laser subassembly that has a sticker reading
something like: "Does not Comply with 21 CFR". All this means is that since
the laser was mounted inside another piece of equipment and would not
normally be exposed except during servicing, it does not meet all the safety
requirements for a laser of its CDRH classification such as electrical
interlocks, turn-on delay, or beam shutter. This label doesn't mean it is any
more dangerous than another laser with similar specifications as long as proper
precautions are taken - such as adding the missing features if using the laser
for some other purpose!

(From: Johannes Swartling (Johannes.Swartling@fysik.lth.se).)

It is not the laser in itself that is given a class number, but the whole
system. A system which is built around a very powerful laser can still be
specified as Class I, if there is no risk of injury when operating the system
under normal conditions. For example, CD players are of class I, but the
(IR) laser diode may in itself be powerful enough to harm the eye. CD players
are designed so that the laser light won't escape the casing.

When it comes to laser safety and exposure levels the regulations are fairly
complicated and I will not go into details. Basically, there are tables with
'safe' levels of exposures. The exposure has to be calculated in a certain way
which is unique to each case, depending on among other things: laser power,
divergence, distance, wavelength, pulse duration, peak power, and exposure
time.. Although it is true that near infrared lasers are potentially more
dangerous than visible because you can't see the radiation, it is incorrect to
say that it must be, say, Class III. The level of exposure may be so low that
it can be a Class I (note that Class II lasers are always visible though, so
infrared lasers are either of Class I or Class III or higher).

(From: John Hansknecht (vplss@lasersafetysystems.com).)

OSHA STD-01-05-001 - PUB 8-1.7 - Guidelines for Laser Safety and Hazard Assessment is an "open source" release of the ANSI Z136.1-1986
standard. It is not as up to date as the present ANSI standard (ANSI Z136.1-2009), but it's close. The ANSI standard is considered to be the
authoritative guide for safe work practices and would be a better source
than a University safety manual. The key point to understand is if a laser
accident ever occurs and a lawsuit ensues, the lawyers will be checking to
see if the facility was following the "recognized best work practices".

While many of the partial circuits and complete schematics in this document
can and have been used in commercial laser products, important safety
equipment has generally been omitted to simplify their presentation. These
range from simple warning labels for low power lasers (Class I, II, IIIa) to
keyswitch and case interlocks, beam-on indicators, and other electrical and
mechanical safety devices for higher power lasers. Laser safety is taken
very seriously by the regulatory agencies. Each classification has its own
set of requirements.

The following brief summary is just meant to be a guide for personal projects
and experimentation (non-commercial use) - the specifics for each laser class
may be even more stringent:

For diode lasers and HeNe lasers outputting 5 mW or less (Classes 1, II,
IIIa), packaging to minimize the chances of accidental exposure to the beam
and standard laser warning labels should be provided.

Where the case can be opened without the use of tools, interlocks which
disable the beam are essential to prevent accidental exposure to laser
radiation (Class IIIa and above). Their activation should also remove
power and bleed off any dangerous voltages (ALL HeNe and argon/krypton
lasers).

Aside from their essential safety function, laser warning or danger stickers
DO add something in the professional and high-tech appearance department.
Companies selling laser accessories will likely offer genuine CDRH approved
stickers. If you are selling any laser based equipment, you'll need them (and
a lot more). For hobbyist, some semi-standard unofficial samples can be found
in the next section.

Edit the labels for your specific laser if necessary. If your equipment is
just a laser and not something containing a laser, remove the word 'product'.
These were created with MSPAINT. I like to use LVIEWP for format conversion
(e.g., .gif->.bmp and vice-versa), filtering, and other simple processing of
graphics and pictures. (The version of LVIEWP I used is shareware but may no
longer be available from the major download sites. There is now a much
expanded commercial product which I haven't tested.) The font is: Arial Bold.
Each of the labels is about 800 x 525 pixels. The result will be about 1.33"
x 0.87" on a 600 dpi color printer or 2.66" x 1.74" on a 300 dpi printer. To
use a 300 dpi printer to produce the same size labels, processing the image
with a 2x2 averaging filter and then subsampling (resizing or scaling) by 2:1
works fairly well.

Many laser companies and some laser organizations sell laser warning, danger,
and aperture signs. One example is:
Laser Institute of America.
They sell a number different types of laser safety warning signs in their
on-line store.

Coherent, Inc. currently has an
offer of free personalized Class IV laser danger signs if you just go to their
Danger Sign Request Page and fill out the form. (If this
link doesn't work, go to Coherent's homepage, "Products", "Lasers", and there
should be a link from there.) It is certainly worth taking
advantage of this offer but please don't abuse the privilege by requesting
too many! I requested one and it arrived in about 6 weeks - a most spiffy
8" x 11" plastic laminated card and the perfect addition to any laser lab!

Or, if you really want people to get the point, try
Big Scary Laser
Warning Sign. Given the graphic, you might want to edit it to
read something like: "Do Not Allow Laser Beam to Contact Remaining Intact
Parts of Body". :)

I'm not a lawyer, but this is what I have learned in doing laser shows for
quite a while.

Please don't give the legislators ideas. Sales of lasers are unregulated
except for medical and laser show systems, and a few systems under export
controls. For all other systems, you just have to register as a manufacturer
if you're making them for public sales and submit your product for
compliance, and maintain records of who it was initially sold to in case
there is a need for a recall. Nothing now on the federal books prohibits
sale unless it is:

Out of safety compliance.

An imported system not in compliance and not registered or not imported
under a investigational document with customs and CDRH. Lasers thus imported
must be destroyed or returned to country of origin when not needed anymore.

A medical laser based system.

A military laser, especially a military laser that has been exempted from
CDRH compliance by the Secretary of Defense. Those lasers must be rendered
non-functional after leaving the military. All those nice range finders
dumped on the market lately are non-compliant and were supposed to be
smashed.

I can sell you a laser system for public show use but you can not run it
in public without a variance. BTW, it's not considered an entertainment laser
till it's integrated into the projector or declared as such on a variance
application or manufactures registration.

If it's manufactured in the USA and domestically certified laser of any
class, you can own it no matter what. The violation occurs if you do a public
display or show with it, regardless if for profit or not, and it
exceeds Class IIIa. I can legally buy a megawatt laser if it's off the shelf
technology and other then the manufacturer having to have the initial
customer's name for recalls and modifications required to meet safety
specs, no further paperwork is required, except in 4 states that
require registration - Texas, New York, New Jersey, and Massachusetts. If I'm
not in one of these states, I can then sell my megawatt laser to anyone, even
a underage kid. I dont have to notify anybody either. I can toss it into the
trash or disassemble it. I can build all the prototypes I want as well,
provided I do not put them into commercial sale. The manufacturer will
sell me a laser with all the CDRH requirements met, what I do with it
after that point is my business, as long as I do not resell it as a
commercial product.

If someone wants to send his prototype laser to the USA, he can do it
legally provided: (1) the receiver and he fill out a bonding document
that says its not compliant, coming in for test, be held by the
receiver as if it were under customs bond, tested by the receiver,
then it is leaving to go back where it came. It will be tracked by
Customs and CDRH and fees may need to be paid. It may not be used as
a public display device during this period. It's here for technical
test and evaluation, period!

Or he may simply register as a manufacturer, file the paperwork certifying
he's built the unit to 21 CFR 1040 (CFR = code of federal regulations)
compliance specs, then assign it a serial number, keep the required
records and sell it as a laser product. It takes about 3 to 6 months for
the paperwork, but actually just costs the postage to get the forms
and send in the documents. He can then sell as many as he likes,
provided he does the documentation. If after introduction to sale, it
is inspected and a fault is found, then he needs to declare his
customer list and provide a scheme to modify it, retrofit it, or recall
it. The paperwork isn't that bad, it just takes a bit of time for a tiny
overworked federal agency to do its duties. Once you get a compliance
or registration number based on your documentation proving you meet the
rules, you can start selling.

If you already have a unit that has a compliance or registration number, you're
fine. All other units are supposed to have a OEM sticker on them, saying its
intended for use in a approved application device only. Possession of those
for private experiments is not a issue.

When you sell a used unit or advertise it for sale, you are required to
include a copy of the warning sticker in your brochure or Web site, even
for class I or class II lasers, and warn the potential buyer of its hazards
in writing, and include any manuals or other safety related gear that was
manufactured for that laser.

You're also supposed to inspect your units for CDRH compliance at least once a
year or when you buy it, and correct any problems you find, but you don't have
to report doing that.

So basically no rules about anything that is not medical, military or foreign.

NOW, where the crap hits the fan is at the state level. New York records the
serial number of all lasers and requires licensed operators, transferring a
laser in NY above class II to another citizen of NY without reregistering the
unit is an offense. Transporting a laser through NY or selling it out of state
from NY is not however a offense. Texas and Arizona have user fees to pay for
their states radiologic safety programs, etc. I'm told by a friend that AZs
fees are quite steep, on the order of $1,500 a year for large industrial
lasers and that AZ inspects laser shows rather thoroughly. Other states may
vary, but generally unless they have made misuse of pointers a issue, there
are no worries except in NY and AZ. Possession is not illegal and they don't
deny permits to register in those states. However, they may disqualify a
person who fails to pass the test.

As far as the national fire code goes, there is one line in the code requiring
a evaluation of the system so the beam doesn't terminate on anything that
could cause a building fire.

I'm sure a few other states have rules, but most don't, and law enforcement
types have much better things to do with their time unless its medical and
misused or malfunctioning, misused as a weapon or in a unsafe manner in public
or the workplace, used in a outdoor laser show going into airspace, or
exported to a foreign country where it would enhance their military or
technology programs.

In all fairness, New York's licensing system includes one heck of a safety
program, including retinal photos of high power laser operators as part of an
ongoing safety study. I doubt they care about what you do in your basement,
provided you're the only one who can be exposed to laser energy.

As long as you don't use the laser for public displays or shows, put it where
the general public can access more then the Class I exposure limits, don't do
anything that risks illuminating an aircraft or other vehicle and don't
practice medicine (including general surgery and dentistry!) with it, the sky
is the limit. You can legally use some Class II lasers and some Class IIIa
lasers in public, as long as you comply with the safety instructions that came
with the device. This is for the US. However, if you wish to do laser shows,
other rules apply. See the sections starting with:
Some Basic Info on Light Show Lasers for
more info.

So have fun and don't worry, unless your aiming it outside or doing public
laser shows or playing doctor or are exporting.

Where you plan to offer a product commercially, there are very specific
requirements - and equally severe penalties for non-compliance!

(From: Steve Roberts (osteven@akrobiz.com).)

"CDRH has fangs and will use them. I have a friend who sold helium-neon laser
power supply kits without stickers, certification, and registering. His legal
bills alone were $8,000, not to mention the hefty fine his company paid."

CDRH will gladly send out a complete copy of the guidelines for manufacturing
laser systems free of charge if you request it. Ask them to throw in the
laser show stuff as it makes even more interesting reading. The hardcopy from
CDRH will include the exposure tables and how to calculate MPEs, etc. It's
free to all US citizens and probably free to overseas corporations as well.

For a more interactive experience, spend an afternoon (more or less) starting
at the CDRH Device
Advice page. According to their blurb: "Device Advice is set up with
pages that describe these procedures and link you to the appropriate documents
on the CDRH Homepage such as guidance documents, databases, and manuals that
will both assist in meeting marketing requirements and answer many questions
you may have."

Here are some answers from CDRH to common questions about laser safety
regulations, particularly with respect to public laser shows and classroom
demonstration. However, although these may be thought of being from the
horse's mouth, you should take them with a grain of laser crystal. In the end,
verbal statements aren't going to hold much weight and you may indeed be
held responsible should the information turn out not to be entirely accurate.

Disclaimer: Long-winded post follows. I do not work for the CDRH. I
have no hidden agenda to either praise or pan the government. I am
not a laser expert. Do not read post while operating heavy machinery.
Not intended for children under the age of 3. no deposit, no return.

With all the discussion about CDRH rules concerning lasers
in schools, laser shows for demonstration purposes, etc., I
decided to try calling the CDRH directly. (Insert ominous music here!)
I called 1-800-638-2041 and got transferred to Walter Snefko at
extension 120. Unfortunately, he was unclear on several of my
questions, and admitted that the real laser device gurus were only
reachable by dialing the non-toll-free number. (Figures!)

So with no concern for daytime long distance rates I took his
suggestion and dialed 1-240-276-0120. Having been primed with
information from Walter, I then asked for Jerry Dennis at extension
135. Jerry was available, and was *AMAZINGLY* helpful. We spoke for
about 15 minutes, and he encouraged me to call again if I had further
questions. He also said that two others in the office, Dale Smith
(extension 147) and Frank Mackson (extension 145) were even more
familiar with CDRH rules as they applied to laser shows. He said that
Dale Smith, in particular, was a very valuable resource.

OK, here's what I asked:

Question: If a laser show has less than 5 mW output power, do you
have to have a variance? (I know, we've talked about it here before, but
since I was at the horse's mouth already it seemed a good time to ask!)

Answer: No. If you are under 5 mW you do NOT need a variance and you
can do whatever you want. (This was correctly pointed out by others
here on alt.lasers before. I was really just warming up with this
question.) However, he *DID* caution me to remember that 5 mW can
still be an eye hazard. Basically he said to avoid sending static
beams into the audience. I pressed him about scanned beams and the
like, and he temporized. I gather that he knew that they really don't
have jurisdiction here because of the power level, but he wanted to
discourage blatantly stupid behavior nonetheless.

Question: If a laser puts out more than 5 mw, but through the use
of a beamsplitter, diffraction grating, filter, or other device the resulting
beams that exit the projector are EACH less than 5 mw, is the show
still considered "under 5 mw" and thus exempt from a variance?

Answer: Yes. As long as the beams that exit the device (projector,
housing, etc) are each less than 5 mw, and the beams are separated far
enough apart such that it is not possible for multiple beams to enter
the same pupil, then the entire device is exempt. Thus, you can have
a 20 mw laser putting out, say, 8 beams from a diffraction grating,
and as long as none of the beams are over 5 mw the entire setup is
exempt.

As an aside, he went into considerable detail as to how these beams
would be measured for intensity. The basic standard is to use the
standardized pupil diameter (listed in the CDRH main document - I
forgot to write it down) and measure the power at a minimum distance
of 20 centimeters (!) from the apparent source of the laser light.
(i.e., the window on the projector housing, the aperture of the scan
head, or the output coupler of the laser if the other two do not
apply.) The pupil size used for the calculation changes if the
environment is such that it would be likely that audience members
would be using binoculars. Binoculars?!? I asked?: "Yeah, like in a
large arena - folks typically bring a set to make it easier to see the
stage." He floored me with that one, but it makes sense if you think
about it.

Question: How are laser demonstrations - especially in classrooms -
handled? Are they subject to CDRH rules?

Answer: No. The CDRH deals with commercial products only: either
laser systems that are assembled to be sold to the public, or laser
shows that are created (designed) to be "sold" as a "product" to an
audience. Laser shows that are not part of a commercial endeavor are
NOT subject to CDRH rules, no matter what power levels are involved.
This was AMAZING to hear as it runs counter to just about everything
I've read here and elsewhere, so I questioned Jerry at length about it.

Basically, he said that there are white areas, black areas, and grey
areas. The white would be what you do in your basement for you and
your family, friends, or neighbors. This includes classrooms (see
below). The black would be when you set up a show at a local
auditorium and charge admission. The grey area is when you have a
show that could be considered commercial in nature, even if you choose
not to charge admission. Examples include doing a laser show for free
at some other event, when it would normally be customary to pay to
gain admittance to that event. However, he did make it clear that in
order for the CDRH to have legal authority, the situation must involve
COMMERCE of some sort. Thus, volunteering to show off your Class IV
lasers in the church basement after the service is over (call it your
Sunday afternoon laser club?) would be an exempted laser show.

Displays in a classroom were one case that I specifically asked about.
I asked about the case of a private citizen volunteering to do a
display for the class, as well as the case of a teacher doing a
demonstration for his class. In either case, because there is no
commerce involved, the CDRH does not have jurisdiction and a variance
is not required NO MATTER WHAT POWER LEVEL LASER IS USED.

Now, before you start thinking that this is a gaping legal loophole,
Jerry did inform me that there is an entirely different set of
guidelines that are normally applied to classrooms.

The American National Standards Institute has released a document
dealing with what are considered acceptable practices for using lasers
in school environments. The standard is ANSI Z 136.5, and evidently
it is available from the Laser Institute of America for a nominal fee.
(No, I haven't checked yet.) Nearly all college environments dealing
with class 3B and Class IV lasers are expected to adhere to this
standard according to Jerry.

He also pointed out that compliance with the ANSI standard is not
required by the CDRH. (Though it might well be required by the school
district, as well as local or state laws dealing with lasers) It is
simply an example of what a legal court of law would accept as a
"reasonable standard of conduct" for the safe operation of a laser in
a classroom environment. (Translation: if you do not follow the
standard, no one from the CDRH will shut you down, fine you, or take
you to jail, but if anyone ever gets hurt at your show and decides to
sue you, they have a better chance of winning if you failed to follow
the guidelines set down in the standard.) Short answer is that I'm
going to at least have a look at that standard before I consider
taking any of my lasers into a classroom.

I have to admit that I was a bit concerned about calling the CDRH. I
wondered if they would take the time to talk to an "enthusiast" that
really didn't have much intention of ever going commercial. I was
afraid I'd be accused of wasting their time. I was also a little
afraid that they would read me the riot act for even asking some of
these questions.

What I found out is that the folks there are VERY helpful. I'm not
kidding, this Jerry Dennis fellow sounds like a great guy! He took
the time to answer my questions at great length, and always pointed
out that even if certain circumstances were not under CDRH control it
would still be foolish to ignore the safety precautions they promote.
He really was easy to talk to, and was quite supportive of my hobby.
Despite the fact that he was clearly used to speaking to engineers and
Ph.D.s about laser physics, he was able to speak at my level about
everything we talked about. He also had some good information about
laser safety. (Hey, how about that? A government employee that
actually gives great customer service!) I'll probably call him (or
maybe Dale Smith) again in the future.